[ICE]: assertion failed: self_ty.is_some()

[matthiaskrgr]

auto-reduced (treereduce-rust):

trait Trait{
    fn bar4();
}
impl Trait for () {
    reuse Trait::<> as bar4;
}

original:

#![feature(fn_delegation)]
trait Trait{
    fn bar4();
}
impl Trait for () {
    reuse Trait::<> as bar4;
}

fn main() {}

Version information

rustc 1.96.0-nightly (b90dc1e59 2026-03-04)
binary: rustc
commit-hash: b90dc1e597db0bbc0cab0eccb39747b1a9d7e607
commit-date: 2026-03-04
host: x86_64-unknown-linux-gnu
release: 1.96.0-nightly
LLVM version: 22.1.0

Possibly related line of code:

rust/compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

Lines 626 to 638 in b90dc1e

	if generics.parent.is_some() {
	// The parent is a trait so it should have at least one
	// generic parameter for the `Self` type.
	assert!(!parent_args.is_empty())
	} else {
	// This item (presumably a trait) needs a self-type.
	assert!(self_ty.is_some());
	}
	} else {
	assert!(self_ty.is_none());
	}
	let arg_count =

Command:
/home/matthias/.rustup/toolchains/master/bin/rustc

Program output

error[E0423]: expected function, found trait `Trait`
 --> /tmp/icemaker_global_tempdir.kxGqBK3eThhO/rustc_testrunner_tmpdir_reporting.Mv9iDPBjanfo/mvce.rs:6:11
  |
6 |     reuse Trait::<> as bar4;
  |           ^^^^^^^^^ not a function

error[E0658]: functions delegation is not yet fully implemented
 --> /tmp/icemaker_global_tempdir.kxGqBK3eThhO/rustc_testrunner_tmpdir_reporting.Mv9iDPBjanfo/mvce.rs:6:5
  |
6 |     reuse Trait::<> as bar4;
  |     ^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: see issue #118212 <https://github.com/rust-lang/rust/issues/118212> for more information
  = help: add `#![feature(fn_delegation)]` to the crate attributes to enable
  = note: this compiler was built on 2026-03-04; consider upgrading it if it is out of date

error[E0601]: `main` function not found in crate `mvce`
 --> /tmp/icemaker_global_tempdir.kxGqBK3eThhO/rustc_testrunner_tmpdir_reporting.Mv9iDPBjanfo/mvce.rs:7:2
  |
7 | }
  |  ^ consider adding a `main` function to `/tmp/icemaker_global_tempdir.kxGqBK3eThhO/rustc_testrunner_tmpdir_reporting.Mv9iDPBjanfo/mvce.rs`


thread 'rustc' (2422681) panicked at /rustc-dev/b90dc1e597db0bbc0cab0eccb39747b1a9d7e607/compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs:632:17:
assertion failed: self_ty.is_some()
stack backtrace:
   0:     0x7f5b9175b73b - <<std[82796cbe7e14e686]::sys::backtrace::BacktraceLock>::print::DisplayBacktrace as core[723ec164f49f73a3]::fmt::Display>::fmt
   1:     0x7f5b91e22948 - core[723ec164f49f73a3]::fmt::write
   2:     0x7f5b91772746 - <std[82796cbe7e14e686]::sys::stdio::unix::Stderr as std[82796cbe7e14e686]::io::Write>::write_fmt
   3:     0x7f5b91731818 - std[82796cbe7e14e686]::panicking::default_hook::{closure#0}
   4:     0x7f5b9174eba3 - std[82796cbe7e14e686]::panicking::default_hook
   5:     0x7f5b9073866c - std[82796cbe7e14e686]::panicking::update_hook::<alloc[774c2fc5b8157a70]::boxed::Box<rustc_driver_impl[cc40153740e5fbc5]::install_ice_hook::{closure#1}>>::{closure#0}
   6:     0x7f5b9174ee82 - std[82796cbe7e14e686]::panicking::panic_with_hook
   7:     0x7f5b9173190a - std[82796cbe7e14e686]::panicking::panic_handler::{closure#0}
   8:     0x7f5b91728609 - std[82796cbe7e14e686]::sys::backtrace::__rust_end_short_backtrace::<std[82796cbe7e14e686]::panicking::panic_handler::{closure#0}, !>
   9:     0x7f5b9173333d - __rustc[eda097aff079559a]::rust_begin_unwind
  10:     0x7f5b8e31c61c - core[723ec164f49f73a3]::panicking::panic_fmt
  11:     0x7f5b8de00022 - core[723ec164f49f73a3]::panicking::panic
  12:     0x7f5b92513965 - <dyn rustc_hir_analysis[e0c4acfd5fb67af7]::hir_ty_lowering::HirTyLowerer>::lower_generic_args_of_path::{closure#0}
  13:     0x7f5b908d004f - rustc_hir_analysis[e0c4acfd5fb67af7]::delegation::get_delegation_user_specified_args
  14:     0x7f5b908cf32f - rustc_hir_analysis[e0c4acfd5fb67af7]::delegation::inherit_sig_for_delegation_item
  15:     0x7f5b911eadf2 - rustc_query_impl[eb5d4e66a16195f6]::query_impl::inherit_sig_for_delegation_item::invoke_provider_fn::__rust_begin_short_backtrace
  16:     0x7f5b927a480c - rustc_query_impl[eb5d4e66a16195f6]::execution::try_execute_query::<rustc_data_structures[b07b151140ce4279]::vec_cache::VecCache<rustc_span[da1975f195527d0f]::def_id::LocalDefId, rustc_middle[9b0503ad92008f82]::query::erase::ErasedData<[u8; 16usize]>, rustc_middle[9b0503ad92008f82]::dep_graph::graph::DepNodeIndex>, false>
  17:     0x7f5b911eae5a - rustc_query_impl[eb5d4e66a16195f6]::query_impl::inherit_sig_for_delegation_item::execute_query_non_incr::__rust_end_short_backtrace
  18:     0x7f5b9250d293 - <dyn rustc_hir_analysis[e0c4acfd5fb67af7]::hir_ty_lowering::HirTyLowerer>::lower_ty
  19:     0x7f5b9252262f - <dyn rustc_hir_analysis[e0c4acfd5fb67af7]::hir_ty_lowering::HirTyLowerer>::lower_fn_ty
  20:     0x7f5b92104cfc - rustc_hir_analysis[e0c4acfd5fb67af7]::collect::fn_sig
  21:     0x7f5b92104466 - rustc_query_impl[eb5d4e66a16195f6]::query_impl::fn_sig::invoke_provider_fn::__rust_begin_short_backtrace
  22:     0x7f5b92108ba0 - rustc_query_impl[eb5d4e66a16195f6]::execution::try_execute_query::<rustc_middle[9b0503ad92008f82]::query::caches::DefIdCache<rustc_middle[9b0503ad92008f82]::query::erase::ErasedData<[u8; 24usize]>>, false>
  23:     0x7f5b92108093 - rustc_query_impl[eb5d4e66a16195f6]::query_impl::fn_sig::execute_query_non_incr::__rust_end_short_backtrace
  24:     0x7f5b927ab5eb - rustc_hir_analysis[e0c4acfd5fb67af7]::check::compare_impl_item::check_method_is_structurally_compatible
  25:     0x7f5b927b0a47 - rustc_hir_analysis[e0c4acfd5fb67af7]::check::compare_impl_item::compare_impl_item
  26:     0x7f5b927b05e9 - rustc_query_impl[eb5d4e66a16195f6]::query_impl::compare_impl_item::invoke_provider_fn::__rust_begin_short_backtrace
  27:     0x7f5b92b51799 - rustc_query_impl[eb5d4e66a16195f6]::execution::try_execute_query::<rustc_data_structures[b07b151140ce4279]::vec_cache::VecCache<rustc_span[da1975f195527d0f]::def_id::LocalDefId, rustc_middle[9b0503ad92008f82]::query::erase::ErasedData<[u8; 1usize]>, rustc_middle[9b0503ad92008f82]::dep_graph::graph::DepNodeIndex>, false>
  28:     0x7f5b92b5128e - rustc_query_impl[eb5d4e66a16195f6]::query_impl::compare_impl_item::execute_query_non_incr::__rust_end_short_backtrace
  29:     0x7f5b92b54740 - rustc_hir_analysis[e0c4acfd5fb67af7]::check::check::check_item_type
  30:     0x7f5b92b52226 - rustc_hir_analysis[e0c4acfd5fb67af7]::check::wfcheck::check_well_formed
  31:     0x7f5b92b52207 - rustc_query_impl[eb5d4e66a16195f6]::query_impl::check_well_formed::invoke_provider_fn::__rust_begin_short_backtrace
  32:     0x7f5b92b51799 - rustc_query_impl[eb5d4e66a16195f6]::execution::try_execute_query::<rustc_data_structures[b07b151140ce4279]::vec_cache::VecCache<rustc_span[da1975f195527d0f]::def_id::LocalDefId, rustc_middle[9b0503ad92008f82]::query::erase::ErasedData<[u8; 1usize]>, rustc_middle[9b0503ad92008f82]::dep_graph::graph::DepNodeIndex>, false>
  33:     0x7f5b92b514fb - rustc_query_impl[eb5d4e66a16195f6]::query_impl::check_well_formed::execute_query_non_incr::__rust_end_short_backtrace
  34:     0x7f5b92b4e3fd - rustc_hir_analysis[e0c4acfd5fb67af7]::check::wfcheck::check_type_wf
  35:     0x7f5b92b4e307 - rustc_query_impl[eb5d4e66a16195f6]::query_impl::check_type_wf::invoke_provider_fn::__rust_begin_short_backtrace
  36:     0x7f5b93086797 - rustc_query_impl[eb5d4e66a16195f6]::execution::try_execute_query::<rustc_middle[9b0503ad92008f82]::query::caches::SingleCache<rustc_middle[9b0503ad92008f82]::query::erase::ErasedData<[u8; 1usize]>>, false>
  37:     0x7f5b93086569 - rustc_query_impl[eb5d4e66a16195f6]::query_impl::check_type_wf::execute_query_non_incr::__rust_end_short_backtrace
  38:     0x7f5b9289d915 - rustc_hir_analysis[e0c4acfd5fb67af7]::check_crate
  39:     0x7f5b91f42e93 - rustc_interface[5358b22c06b9f13b]::passes::analysis
  40:     0x7f5b9308835c - rustc_query_impl[eb5d4e66a16195f6]::execution::try_execute_query::<rustc_middle[9b0503ad92008f82]::query::caches::SingleCache<rustc_middle[9b0503ad92008f82]::query::erase::ErasedData<[u8; 0usize]>>, false>
  41:     0x7f5b930880bb - rustc_query_impl[eb5d4e66a16195f6]::query_impl::analysis::execute_query_non_incr::__rust_end_short_backtrace
  42:     0x7f5b9305bdad - rustc_interface[5358b22c06b9f13b]::interface::run_compiler::<(), rustc_driver_impl[cc40153740e5fbc5]::run_compiler::{closure#0}>::{closure#1}
  43:     0x7f5b930322be - std[82796cbe7e14e686]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[5358b22c06b9f13b]::util::run_in_thread_with_globals<rustc_interface[5358b22c06b9f13b]::util::run_in_thread_pool_with_globals<rustc_interface[5358b22c06b9f13b]::interface::run_compiler<(), rustc_driver_impl[cc40153740e5fbc5]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  44:     0x7f5b93032920 - <std[82796cbe7e14e686]::thread::lifecycle::spawn_unchecked<rustc_interface[5358b22c06b9f13b]::util::run_in_thread_with_globals<rustc_interface[5358b22c06b9f13b]::util::run_in_thread_pool_with_globals<rustc_interface[5358b22c06b9f13b]::interface::run_compiler<(), rustc_driver_impl[cc40153740e5fbc5]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[723ec164f49f73a3]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  45:     0x7f5b930337ac - <std[82796cbe7e14e686]::sys::thread::unix::Thread>::new::thread_start
  46:     0x7f5b8caa598b - <unknown>
  47:     0x7f5b8cb29a0c - <unknown>
  48:                0x0 - <unknown>

error: the compiler unexpectedly panicked. This is a bug

note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: please make sure that you have updated to the latest nightly

note: rustc 1.96.0-nightly (b90dc1e59 2026-03-04) running on x86_64-unknown-linux-gnu

query stack during panic:
#0 [inherit_sig_for_delegation_item] inheriting delegation signature
#1 [fn_sig] computing function signature of `<impl at /tmp/icemaker_global_tempdir.kxGqBK3eThhO/rustc_testrunner_tmpdir_reporting.Mv9iDPBjanfo/mvce.rs:5:1: 5:18>::bar4`
#2 [compare_impl_item] checking assoc item `<impl at /tmp/icemaker_global_tempdir.kxGqBK3eThhO/rustc_testrunner_tmpdir_reporting.Mv9iDPBjanfo/mvce.rs:5:1: 5:18>::bar4` is compatible with trait definition
#3 [check_well_formed] checking that `<impl at /tmp/icemaker_global_tempdir.kxGqBK3eThhO/rustc_testrunner_tmpdir_reporting.Mv9iDPBjanfo/mvce.rs:5:1: 5:18>` is well-formed
#4 [check_type_wf] checking that types are well-formed
#5 [analysis] running analysis passes on crate `mvce`
end of query stack
error: aborting due to 3 previous errors

Some errors have detailed explanations: E0423, E0601, E0658.
For more information about an error, try `rustc --explain E0423`.

@rustbot label +F-fn_delegation

[GokhanKabar]

I've opened a PR to fix this: #153421

The ICE was caused by get_delegation_user_specified_args calling lower_generic_args_of_path with self_ty = None when the child segment resolves to a trait (which has has_self = true). The fix provides self_ty only during error recovery (when the child segment is not a function).